1. Field of the Invention
The present invention relates to a sheet transport apparatus for transporting a sheet and detecting the sheet and to an image forming apparatus having the sheet transport apparatus in the apparatus body.
2. Description of the Related Art
For example, known image forming apparatuses for forming an image on a sheet include a sheet transport apparatus that transports a sheet. Examples of the image forming apparatus include a copier, a printer, a facsimile, and a multi-function apparatus combining these functions.
Some sheet transport apparatuses include a sheet detection sensor (refer to, for example, Japanese Patent Laid-Open No. 10-87115). The sheet detection sensor detects, for example, a transported sheet, the shape of sheet curl, and a sheet skew. An electro photographic copier, which is an example of the image forming apparatus, detects the transportation of a sheet fed from a sheet cassette using a sheet detection sensor so as to control the operations of an image forming unit and a heat fusing unit disposed downstream of a transport path on the basis of the detected timing of the sheet detection sensor.
Such sheet detection sensors include, for example, a photo interrupter sensor 858 shown in FIG. 14. The photo interrupter sensor 858 includes a rotatable flag 851 disposed for temporarily interrupting the passage of a sheet S and a photo interrupter 853 to detect the interruption of a detection light beam 853a. The flag 851 is brought into contact with a stopper 852b by a spring 852a, and therefore, the rotation of the flag 851 is restricted.
In the photo interrupter sensor 858, when the transported sheet S hits against the flag 851, the flag 851 rotates about a fulcrum 851a. Accordingly, a light interrupting portion 851b interrupts the detection light beam 853a so that the photo interrupter sensor 858 detects the arrival of the sheet S to output an electric signal. The electric signal is sent to a controller (not shown) that carries out overall control of the copier.
In the image forming apparatus, if a sheet is curled, the image quality and the stacking performance of output sheets may deteriorate. Therefore, information whether a sheet is curled or not is significantly important information. To determine the shape of a curl, a method has been proposed in which the passage position (position in a direction perpendicular to the transport surface of the sheet) of the leading edge of the sheet is detected. In this method, a plurality of photo interrupter sensors having flags of different lengths are provided to detect the passage of the sheet S and the shape of a curl of the sheet S. In FIG. 15, three photo interrupter sensors 858a, 858b, and 858c include flags 851c, 851d, and 851e, respectively.
For example, if the three photo interrupter sensors 858a, 858b, and 858c detect a sheet at the same time, it is determined that the sheet S is curled downwards, as shown in FIG. 15. If the photo interrupter sensor 858a, which has the longest flag, detects the sheet S first, it is determined that the sheet S is curled upwards.
Additionally, like the curl of a sheet, the skew of a sheet significantly decreases the image quality. Accordingly, detecting a skew is also an important factor for the image forming apparatus. To detect a skew, as shown in FIG. 16, three photo interrupter sensors 858 are arranged in a direction perpendicular to a sheet feed direction so that the skew of the sheet can be detected from a difference among detection timings of the three photo interrupter sensors 858.
However, as shown in FIG. 17, when only an end portion F at the leading edge of the sheet is curled, the leading edge of the end portion F is located at a position shifted towards the upstream of the sheet feed direction. Accordingly, in the arrangement of the photo interrupter sensors shown in FIG. 16, a curl may be mistakenly detected as a skew. To solve this problem, a plurality of groups including the three photo interrupter sensors 858a, 858b, and 858c having respective flags 851c, 851d, and 851e of different lengths, as shown in FIG. 15, can be provided at positions as shown in FIG. 16.
Although the above-described photo interrupter sensor 858 has a simple structure, the photo interrupter sensor 858 has the following problems:
(1) The photo interrupter sensor 858 that includes the photo interrupter 853 requires a large installation space, and therefore, it is difficult to mount the photo interrupter sensors 858 in some installation areas.
In the photo interrupter sensor 858, the flag 851 needs to be mounted separately from the photo interrupter 853 with a precise spacing therebetween. Therefore, at some installation positions of the photo interrupter sensors 858, there may be no space for the photo interrupter 853. Additionally, when the flag 851 and the photo interrupter 853 are mounted on different parts, it is difficult to ensure the precise spacing therebetween.
(2) A chattering phenomenon tends to occur.
The chattering phenomenon refers to a repetitive motion in which, when the flag 851 pushed down by the sheet S returns to the original position due to a force by the spring 852a, the flag 851 hits against the stopper 852b, bounces back, and hits against the stopper 852b again. When the chattering phenomenon occurs, the photo interrupter sensor 858 unstably interrupts the detection light beam 853a, and therefore, the detection timing of the sheet S becomes inaccurate. In known photo interrupter sensors 858, since the flag 851 has the light interrupting portion 851b, the total weight of the flag 851 increases, and therefore, the chattering phenomenon easily occurs. Increasing the spring force of the spring 852a can prevent this phenomenon. However, the flag 851 does not smoothly rotate when pushed by the sheet S. Consequently, when the sheet S is a thin paper sheet, the leading edge of the sheet S may be damaged.
(3) It is difficult to determine the shape of a curl.
As shown in FIG. 15, the following reason makes it difficult for the three photo interrupter sensors 858a, 858b, and 858c to determine the shape of a curl.
For example, to install a plurality of sets of the three photo interrupter sensors 858a, 858b, and 858c, a large installation space is needed, as described in (1), and therefore, it is difficult to mount the photo interrupter sensors 858 in some areas.
Also, in the photo interrupter sensor 858, a spring force of the spring 852a that returns the flag needs to increase in order to prevent the chattering phenomenon described in (2). If the plurality of photo interrupter sensors 858 is arranged, as shown in FIG. 15, the transportation of a thin paper sheet may be blocked.
(4) When the sheet S is skewed, it is difficult to distinguish the skew from a curl of the sheet S.
As shown in FIG. 16, if a plurality of sets of the photo interrupter sensors shown in FIG. 15 are arranged, a thin paper sheet described in (2) may be damaged.